Geometrical construction to derive phase birefringence B(λ) from group birefringence B_g(λ) (black solid line) and a single-wavelength phase measurement B(λ₁) (red circular dot); (a) construction of the tangent T₂(λ) that crosses tangent T₁(λ) at λ_x = (λ₂ − λ₁)/ln(λ₂/λ₁) (dashed blue lines), which allows one to find B(λ₂);when the interval (λ₂ − λ₁)/λ₁ is small, λ_x ≈ (λ₂ + λ₁)/2; (b) construction of B(λ) (dashed red curve) that must fit the tangents T₁(λ) and T₂(λ).